Polyethylene terephthalate and its copolyesters (“PET”) are used extensively in packaging applications, in particular as beverage containers. The conventional PET production process begins with esterification of predominantly terephthalic acid and ethylene glycol, or ester exchange of predominantly dimethyl terephthalate and ethylene glycol at a temperature of about 265° C. The esterification need not be catalyzed. Typical ester exchange catalysts, which may be used separately or in combination, include, zinc, manganese or magnesium acetates, titanium alkoxides, tin esters and/or other such catalyst materials that are well known to those skilled in the art. The resulting mixture, average degree of polymerization (DP) of less than 20, is then subjected to polycondensation in the melt at elevated temperature, for example about 285° C. and above, in the presence of a suitable catalyst until it reaches the desired molecular weight, or Intrinsic Viscosity (IV). Compounds of Sb, Sn, Ge and others have been used as polycondensation catalysts.
There is a trend in the industry to move from the conventional medium activity polycondensation catalysts, such as antimony compounds used at amounts of over 250 ppm Sb, to more active catalysts such as titanium compounds that can be used a much lower amounts, e.g. 5 to 15 ppm Ti. However these more active compounds increase the degradation of the polyester which leads to more by-products such as acetaldehyde (AA) and their precursors. Whether this resin, quenched and cut into pellets, is used directly from the melt polymerization process, or after a solid state polymerization process, when processed into extrusion blown, or injection stretch blown bottles, the level of AA is unacceptably high and influences the taste of the liquid in the bottle.
This problem can be overcome by reducing the polycondensation temperatures, but this also requires that the esterification temperatures also be reduced. This therefore reduces the capacity of the continuous polymerization lines.
The problem of controlling the presence of free AA and AA precursors produced in the melt-phase manufacture was discussed in U.S. Pat. No. 6,559,271 B2. This patent proposes that the amount of free AA and AA precursors can be limited by keeping the reaction temperature during the entire polycondensation step below 280° C. by using a low dosage of a highly active titanium catalyst to limit the residence time of the polymer in the melt-phase manufacture. The addition of an AA inhibitor, or by adding an AA bonding substance, reduces the AA to an acceptable level. One remaining problem associated with this approach is that the process described is necessarily costly with long residence times in esterification to maintain the proscribed temperatures.
There is therefore a need for a process that maintains the standard esterification temperature and throughput, but reduces the temperature used for the polycondensation step for highly active catalysts.